Aer Lingus has promised no flight disruption this weekend as it issued a dramatic ultimatum to cabin crew — work our new rosters or face the sack.

The drastic message is being couriered in a letter from management to 140 staff as the airline continues to pay for leased aircraft to ensure a full schedule.

It will continue to use its €900m (£764m) cash reserves to pay for aircraft, which come with pilots and cabin crew, from Ryanair and other carriers to ensure a full service while its own planes remain grounded.

This means it is paying its own pilots, some of whom earn annual salaries over €250,000 (£212,000), in addition to the leased-in crew — estimated to cost around €20,000 (£16,000) per short-haul aircraft.

A group of its pilots who are now “on reserve” turned up for a march yesterday in support of cabin crew who told the airline they were ready, willing and able to work.

But their appeal did little to appease the airline, which gave them one last chance to sign up to the new rosters that would increase their annual flying hours to 850 a year.

The letter asked them to sign a “form of undertaking” or enter a disciplinary process that means they could be dismissed and replaced. It accused them of failing to comply with their contractual obligations.

The move followed a week of flight chaos that affected more than 4,000 passengers and saw 50-plus flights cancelled.

It is understood that a handful of cabin crew who refused to work the rosters and were going through the disciplinary process before being struck off the payroll have already signed up to the letter of undertaking.